Scientists have disagreed over methods used in a number of high-profile studies that found microplastics throughout the human body. Having been criticised, the researchers who worked on them say that the scientific community must continue to improve on these results. ‘I would not dismiss the papers coming out – they are using the best methods available and the consistency across labs is becoming clear,’ Matthew Campen from the University of New Mexico tells Chemistry World.

The issue was brought to public attention by an investigation in The Guardian that highlighted concerns raised by scientists about these studies. They have written letters to the journals that published the papers and also criticised them on social media. The Guardian reports that critics say ‘the discoveries are probably the result of contamination and false positives’. ‘We fully accept the criticisms, but feel that they are being overinflated,’ says Campen. ‘We had already acknowledged everything that has been discussed in the paper itself.’

The key shortcomings arise from a lack of well-established methods to measure tiny, complex, highly variable microplastic particles. As well as calling out these issues themselves, the researchers are working to establish better methods and standards.

Meanwhile, Marja Lamoree from the Vrije Universiteit Amsterdam, the Netherlands, says that the criticism has strayed into harassment. She tells Chemistry World that she received ‘intimidating and very nasty emails’. After reading them, she was left ‘look[ing] over my shoulder’. ‘It was totally unacceptable.’

No universal method

The Guardian report focused on seven studies that scientists later criticised in letters to the original publishing journals. Two studies involved Campen and colleagues, one finding that micro- and nanoplastic levels in human brain tissue have increased since 1997. Another found microplastics in dog and human testicles, suggesting a connection to sperm count. Another paper, from a team including Lamoree, measured microplastics in blood. Central among the various criticisms are concerns that the plastics discovered might have been accidentally introduced by the scientists themselves.

Dorota Bartczak, nanomaterials science area lead at the National Measurement Laboratory, hosted by LGC in Guildford, UK, notes that infrared and Raman absorption spectroscopic imaging techniques can measure microplastic particles’ size. And mass spectrometric techniques can measure the mass of polymer, explains Bartczak, who has not been the focus of any criticism. Laws and standards define the particles very broadly, for example any plastic particles below 5mm in size, according to the European Chemical Agency, she adds. As such, while there are various techniques in use, none can measure every type of microparticle, Bartczak stresses.

Additionally, lab environments commonly contain microplastics. To account for this, researchers must measure blank samples containing control substances. Such a blank might be filtered ultrapure water put through the same preparation process as the samples of interest. The amount of micro- and nanoplastics detected in the blanks affects the detection limit of any given technique, Lamoree explains. The detection limit determines the smallest amount or mass researchers can reliably claim to detect. ‘We’ve been analysing hundreds of blank samples, and that’s not what many people do,’ she says.

Hard to control

A further challenge that Lamoree has faced came when her team sought to use a standard polystyrene reference material to calibrate their measurements from two different suppliers. They found that the two different materials could vary in the measured response ‘by a factor of 10 sometimes’, she says. That makes it very difficult to use the standards to try to work out the amount of plastic present.

That issue is likely down to different characterisation methods used by different companies, explains Dan Biggerstaff, technical director at LGC Standards, which did not supply Lamoree in this case. ‘If manufacturer A characterises their reference material with technique A, and then a laboratory uses technique B, they’re not going to match. We are in the infancy of this entire research arena – those are problems we’re working to solve.’

Some criticisms focus on challenges faced by one specific technique, pyrolysis gas chromatography mass spectrometry (py-GCMS). The otherwise commonly-used liquid chromatography–mass spectrometry technique isn’t well suited to non-polar microplastics, so py-GCMS is more common.

py-GCMS burns a plastic polymer, releasing component monomers it’s built from and related compounds and then detects them, explains Biggerstaff. It can be quantitative in that it can tell you the total mass of particles, but not how many particles or their size. ‘You could have 50 10µm particles that would have the same mass as, say that one larger particle,’ Biggerstaff says. Critics also warn that it can struggle to differentiate between polyethylene or polyvinylchloride and fats in human tissues. Biggerstaff highlights the need for ‘two different techniques that support the same value before you can start trusting’ figures produced.

After first being criticised in 2022, Lamoree and colleagues highlighted inconsistencies between approaches that different microplastic research groups use. In a review paper they scored different studies’ analytical methods for reliability. The overall measurement issue arises in part because journal editors and peer reviewers often do not have the expertise in microplastic analysis required to judge papers submitted, she says.

Reliable concern

Bartczak and colleagues are trying to help by developing representative test materials and quality control materials. They have also produced a practical guide on how to measure microplastics reliably. ‘Any study that does not report laboratory blank levels, limits of detection and quantification, or appropriate quality control measures should be treated with caution,’ she says. ‘Studies that apply multiple orthogonal analytical approaches and report consistent, converging results provide the greatest confidence.’

The National Measurement Laboratory has developed a platform spanning infrared absorption spectroscopy, mass spectrometry and optical imaging to cover the whole range of relevant particle sizes and types, Bartczak says. Other labs are also doing good work, she stresses. ‘There are many examples of careful, systematic studies that report reliable results,’ Bartczak says. ‘There is broad agreement that microplastics are persistent pollutants of growing global concern, that they accumulate in all environmental compartments, and that they are present in drinking water and food. However, the true scale of the problem and the associated toxicological implications are still being evaluated.’